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Banded iron formation
Banded iron formations (also known as banded ironstone formations or 'BIF's) are distinctive units of that are almost always of age. A typical banded iron formation consists of repeated, thin layers (a few millimeters to a few centimeters in thickness) of silver to black s, either (Fe3O4) or (Fe2O3), alternating with bands of iron-poor s and s, often red in color, of similar thickness, and containing microbands (sub-millimeter) of iron oxides. Some of the oldest known rock formations (having formed ca. ), are associated with banded iron formations. Banded iron formations account for more than 60% of global iron reserves, and can be found in Australia, Brazil, Canada, India, Russia, South Africa, Ukraine, and the United States. Origins .}} The formations are abundant around the time of the , 2,400 million years ago ( ), and become less common after 1,800 with evidence pointing to intermittent low levels of free atmospheric oxygen. new banded iron formations formed that may be associated with the theoretical . in South Africa. The red layers were laid down when Archaean photosynthesizing cyanobacteria produced oxygen that reacted with dissolved iron compounds in the water, to form insoluble iron oxide (rust). The white layers are sediments that settled when there was no oxygen in the water.}} The conventional hypothesis is that the banded iron layers were formed in as the result of released by . The oxygen then combined with dissolved in Earth's oceans to form insoluble iron oxides, which precipitated out, forming a thin layer on the ocean floor, which may have been mud (forming shale and chert). Each band is similar to a , to the extent that the banding is assumed to result from cyclic variations in available oxygen. It is unclear whether these banded ironstone formations were seasonal, followed some in the ocean's or followed some other cycle. It is assumed that initially the Earth started with vast amounts of iron and nickel dissolved in the world's acidic seas. As photosynthetic organisms generated oxygen, the available iron in the Earth's oceans precipitated out as iron oxides. At a suspected where the oceans became permanently oxygenated, small variations in oxygen production produced periods of free oxygen in the surface waters, alternating with periods of iron oxide deposition. Formation Process BIFs occur in two forms, Algoma and Superior-type. Algoma-type Algoma-type are generally smaller in size and formed primarily in the Archean. Algoma-type BIFs are generally found in volcanic rocks in greenstone belts. The formation process involves the chemical precipitation of iron in anoxic environments. When oxidized the iron would precipitate and sink to the bottom of the seafloor. As the oxygen levels continuously shift, the magnetite beds interlayered with amorphous/microcrystalline quartz (i.e. jasper). Superior-type Superior type are the second and larger form of BIFs. They primarily formed during the era, occurring on continental shelves and can be found around the world. Superior types were formed by chemical precipitation in shallow waters, primarily due to the low atmospheric and ocean oxygen levels, resulting in high iron levels in the oceans. Under calm shallow conditions, oxygen released during photosynthesis by blue-green algae, would combine with the iron creating magnetite, which would then sink and deposit on the floor. Snowball Earth , Spain}} Until 1992 it was assumed that the rare, later (younger) banded iron deposits represented unusual conditions where oxygen was depleted locally. Iron-rich waters would then form in isolation and subsequently come into contact with oxygenated water. The hypothesis provided an alternative explanation for these younger deposits. In a Snowball Earth state the continents, and possibly seas at low latitudes, were subject to a severe ice age circa 750 to 580 that nearly or totally depleted free oxygen. Dissolved iron then accumulated in the oxygen-poor oceans (possibly from seafloor hydrothermal vents). Following the thawing of the Earth, the seas became oxygenated once more causing the precipitation of the iron. An alternative mechanism for banded iron formations in the Snowball Earth era suggests the iron was deposited from metal-rich s in the vicinity of ly active s. Alternatively, some geochemists suggest that banded iron formations could form by direct oxidation of iron by microbial . Sudbury Basin impact Banded iron formations in northern were found directly underneath a thick layer of ejecta from the impact. At the time of formation Earth had a single supercontinent called with substantial continental shelves. An (estimated at 10 km across) slammed into waters about 1,000 m deep some 1.85 billion years ago. Computer models suggest that the would have been at least 1,000 metres high at the centre, and 100 metres high about 3,000 kilometres away. Those immense waves and large underwater landslides triggered by the impact stirred the ocean, bringing oxygenated waters from the surface down to the ocean floor. Sediments deposited on the seafloor before the impact, including banded iron formations, contained little if any iron (Fe(III)), but were high in iron (Fe(II)). This Fe(III) to Fe(II) ratio suggests that most parts of the ocean were relatively devoid of oxygen. s deposited after the impact included substantial amounts of Fe(III) but very little Fe(II). This suggests that sizeable amounts of were available to form sediments rich in Fe(III). Following the impact dissolved iron was mixed into the deepest parts of the ocean. This would have choked off most of the supply of Fe(II) to shallower waters where banded iron formations typically accumulated. The geological record suggests that environmental changes were happening in oceans worldwide even before the Sudbury impact. The role of the Sudbury Basin impact in temporarily shutting down banded iron formation accumulation is not fully understood. References Category:Earth